Neutrophils isolated from mammary glands stimulated with a staphylococcal culture filtrate efficiently killed serum-resistant strains of Escherichia coli. This study was extended and it was shown that an infusion of wide ranging numbers (5 X 10(1) to 5 X 10(6)) of the same strains of E coli into a single mammary gland resulted in bacterial growth, which was eliminated following neutrophil infiltration. This elimination occurred before the appearance of any clinical signs. Once bacterial kill had started in the gland, it continued in the milk after withdrawal from the gland. These results offer an explanation of why causative microbial agents cannot be isolated from some cases of clinical mastitis.